Process for preparing a thin layer of ferroelectric material

ABSTRACT

A process for preparing a thin layer made of ferroelectric material based on alkali metal, exhibiting a determined Curie temperature, transferred from a donor substrate to a carrier substrate by using a transfer technique including implanting light species into the donor substrate in order to produce an embrittlement plane, the thin layer having a first, free face and a second face that is arranged on the carrier substrate. The process comprises a first heat treatment of the transferred thin layer at a temperature higher than the Curie temperature, the thin layer exhibiting a multi-domain character upon completion of the first heat treatment, and introducing, after the first heat treatment, protons into the thin layer, followed by applying a second heat treatment of the thin layer at a temperature lower than the Curie temperature to generate an internal electric field that results in the thin layer being made single domain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/FR2019/050356, filed Feb. 18, 2019,designating the United States of America and published as InternationalPatent Publication WO 2019/175487 A1 on Sep. 19, 2019, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1852122, filed Mar. 12, 2018.

TECHNICAL FIELD

The present disclosure relates to a process for preparing a thin layerof ferroelectric material based on alkali metal. More particularly, thedisclosure relates to a preparation process that makes it possible tomaintain or establish the single-domain character of the ferroelectricmaterial in the thin layer of the final product. This preparationprocess is used, for example, for applications in the fields ofmicroelectronics, micromechanics, photonics, etc.

BACKGROUND

A ferroelectric material is a material that possesses an electricalpolarization in the natural state, it being possible for thispolarization to be reversed by applying an external electric field. Theferroelectric domain refers to each continuous region of the material inwhich the polarization is uniform (all the dipole moments are alignedparallel to each other in a given direction).

A ferroelectric material may thus be characterized as “single domain” inthe case where this material consists of a single region in which thepolarization is uniform or “multidomain” in the case where theferroelectric material comprises a plurality of regions havingpolarities that may be different.

Various processes for forming a thin layer of ferroelectric material areknown from the prior art. Such processes may be, for example, molecularbeam epitaxy, plasma sputtering, plasma deposition (laser-pulseddeposition) or application of the SMART CUT® technology in which a thinlayer is transferred from a bulk substrate made of ferroelectricmaterial by splitting at a fragile zone (or embrittlement plane) formedin the bulk substrate by implantation of light species.

The present disclosure relates more particularly to the preparation of aferroelectric thin layer based on alkali metal obtained by applying sucha process.

According to this process, and after the step of transferring a layer,it is often necessary to apply treatments to the thin layer in order toimprove its surface state, its crystal quality, its adhesion to acarrier, or to modify its thickness.

It is a known practice to use, after the step of transferring a layer, aheat treatment step in order to improve the above-mentioned propertiesof the thin layer. Thus, document FR 2863771 teaches a process forpreparing a thin layer that consists mainly of two steps: a step ofpreparing the surface of the thin layer (such as polishing, for example)and a heat treatment step (such as thermal annealing, for example).

In order to better improve the properties of the thin layer, as well asits properties of adhesion with a carrier, it is necessary to perform aheat treatment at a sufficiently high temperature, the aforementionedproperties of the thin layer improving more the higher the temperatureof the treatment. However, in order to obtain a satisfactory result,this very often means exceeding the Curie temperature of the material.

To recall, the Curie temperature is the temperature beyond which thematerial loses its ferroelectric properties. When it falls back belowthe Curie temperature as it cools, the material regains ferroelectricproperties. However, the regained ferroelectric properties are generallydifferent from the initial ones and this may result, in particular, inthe formation of a plurality of ferroelectric domains within the thinlayer, thus giving it a multi-domain character.

This multi-domain character of the ferroelectric material isproblematic, since it makes the characteristics of the materialinhomogeneous, which may affect the performance of the devices that areto be formed on/in the thin layer. This is particularly the case forsurface acoustic wave (SAW) devices, the propagation of the waves beingaffected by the polarity of the piezoelectric material through whichthey pass.

This is why it is generally not recommended to expose a layer made offerroelectric material to a temperature that exceeds its Curietemperature.

Document FR2914492 teaches a process for producing a thin layer offerroelectric material using the SMART CUT® technique. This documentapplies an electric field to the thin layer so as to improve or restoreits ferroelectric properties.

However, to be applied easily, this process requires having electrodeson each of the faces of the thin layer, which is not always the case.

BRIEF SUMMARY

One embodiment of the disclosure provides a process for preparing a thinlayer of ferroelectric material that is based on alkali metal and issingle domain.

A process is disclosed herein for preparing a thin layer made offerroelectric material based on alkali metal, exhibiting a determinedCurie temperature, transferred from a donor substrate to a carriersubstrate using a transfer technique including implanting atomic speciesinto the donor substrate in order to produce an embrittlement plane, thethin layer having a first, free face and a second face that is arrangedon the carrier substrate.

According to the disclosure, the process for preparing the thin layercomprises a first heat treatment of the transferred thin layer at atemperature higher than the determined Curie temperature, the thin layerexhibiting a multi-domain character upon completion of the first heattreatment. The process also comprises introducing, after the first heattreatment, protons into the thin layer, followed by applying a secondheat treatment of the thin layer at a temperature lower than thedetermined Curie temperature in order to generate an internal electricfield that results in the thin layer being made single domain.

According to other advantageous and non-limiting features of thedisclosure, considered alone or according to any technically feasiblecombination:

-   -   the implanted atomic species are hydrogen ions and/or helium        ions;    -   the transfer technique includes joining the donor substrate to        the carrier substrate and detaching the thin layer at the level        of the embrittlement plane;    -   the introduction of protons into the thin layer is achieved by        means of proton exchange;    -   the proton exchange is carried out by immersing at least the        thin layer in a bath of benzoic acid at a temperature typically        between 200 and 300° C. for 10 minutes to 30 hours;    -   the introduction of the protons into the thin layer is achieved        by means of ion implantation or by means of plasma implantation;    -   the first heat treatment is carried out for a duration of        between 30 minutes and 10 hours;    -   the first and the second heat treatments are carried out under        an oxidizing or neutral atmosphere;    -   the second heat treatment is carried out at a temperature less        than 100° C., preferably less than 50° C., or less than 10° C.        from the determined Curie temperature and for a duration of        between 30 minutes and 10 hours;    -   the process for preparing the thin layer comprises a polishing        step applied to the first face of the thin layer;    -   the polishing is carried out after the first heat treatment;    -   the polishing is carried out after the second heat treatment;    -   the polishing is chemical-mechanical polishing;    -   the donor substrate is made of an alkali metal ferroelectric        material based on lithium;    -   the donor substrate is made of LiTaO₃ or of LiNbO₃;    -   the ferroelectric material exhibits a 42°RY crystal orientation;    -   the material of the carrier substrate is silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the disclosure will become apparentfrom the following detailed description of example embodiments, whichdescription is given with reference to the accompanying drawings, inwhich:

FIGS. 1A to 1D show a first embodiment of a process in accordance withthe disclosure;

FIGS. 2A to 2D show a second embodiment of a process in accordance withthe disclosure; and

FIGS. 3A to 3C schematically show a process for preparing a thin layerin accordance with the disclosure.

DETAILED DESCRIPTION

For the sake of keeping the following description simple, the samereferences are used for identical elements or for elements performingthe same function in the various presented embodiments of the process.

The figures are schematic representations which, for the sake oflegibility, are not to scale. In particular, the thicknesses of thelayers are not to scale with respect to the lateral dimensions of theselayers.

The term “coefficient of thermal expansion” used in the rest of thisdescription in relation to a layer or a substrate makes reference to thecoefficient of expansion in a defined direction in the main planedefining this layer or this substrate. If the material is anisotropic,the coefficient value retained will be that of largest amplitude. Thecoefficient value is that measured at room temperature.

The disclosure describes a process for preparing a thin layer 3 made ofa ferroelectric material based on alkali metal, the layer exhibiting adetermined Curie temperature, transferred from a donor substrate 1 to acarrier substrate 7 using a transfer technique including implantinglight species into the donor substrate 1. There are several embodimentsregarding how this transfer may be carried out.

According to a first embodiment shown in FIGS. 1A to 1D, the donorsubstrate 1 is composed of a single-domain bulk block of ferroelectricmaterial based on alkali metal such as KTiOPO₄, KNbO₃, NaNbO₃, KTaO₃,NaTaO₃ and, more particularly, those based on lithium, for example,LiTaO₃ (with a Curie temperature typically between 600° C. and 650° C.)and LiNbO₃ (with a Curie temperature of about 1145° C.). It is entirelypossible to envisage, in the context of the present disclosure, thedonor substrate 1 exhibiting a multi-domain character.

The donor substrate 1 may take the form of a circular wafer ofstandardized size, for example, of 150 mm or 200 mm in diameter.However, the disclosure is not in any way limited to these dimensions orto this form. The donor substrate 1 may have been taken from an ingot offerroelectric materials in such a way as to form the donor substrate 1having a predetermined crystal orientation. The orientation is chosenaccording to the intended application. Thus, it is common practice tochoose a 42° RY orientation in the case where it is desired to use theproperties of the thin layer to form an SAW filter. However, thedisclosure is not in any way limited to a particular crystalorientation.

Whatever the crystal orientation of the donor substrate 1, the processcomprises introducing, into the donor substrate 1, at least one atomicspecies. This introduction may correspond to an implantation, i.e., ionbombardment of the planar face 4 of the donor substrate 1 with speciessuch as hydrogen and/or helium ions.

In a manner known per se, and as is shown in FIG. 1B, the implanted ionshave the role of forming an embrittlement plane 2 delimiting a thinlayer 3 of ferroelectric material to be transferred, which is located onthe planar face 4 side, and another portion 5 forming the rest of thesubstrate.

The nature, the dose of the implanted species, and the implantationenergy are chosen according to the thickness of the layer that it isdesired to transfer and of the physicochemical properties of the donorsubstrate. In the case of a donor substrate 1 made of LiTaO₃, it maythus be chosen to implant a dose of hydrogen of between 1E16 and 5E17at/cm² with an energy of between 30 and 300 keV to delimit a thin layer3 of the order of 200 to 2000 nm.

In a following step, shown in FIG. 1C, the planar face 4 of the donorsubstrate 1 is joined to one planar face 6 of a carrier substrate 7. Thecarrier substrate 7 may have the same dimensions and the same shape asthose of the donor substrate. For reasons of availability and cost, thecarrier substrate 7 is a monocrystalline or polycrystalline siliconwafer. However, more generally, the carrier substrate 7 may consist ofany material, for example, silicon, sapphire or glass, and may have anyshape.

When the process is implemented with a view to producing devices forapplications in the radiofrequency area, it may be advantageous tochoose a carrier substrate 7 that is highly resistive, exhibiting, forexample, a resistivity higher than 1000 ohm·cm. It is also possible tohave a charge-trapping layer, such as a layer of polycrystalline silicona few microns thick, for example, on the carrier substrate 7.

Prior to this joining step, it may be envisaged to prepare the faces ofthe substrates to be joined via a step of cleaning, brushing, drying,polishing or plasma activation.

The joining step may correspond to placing the donor substrate 1 inintimate contact with the carrier substrate 7 by molecular adhesionand/or electrostatic bonding. Optionally, to facilitate the joining ofthe two substrates 1, 7 notably when they are joined by direct bonding,at least one intermediate layer may be formed prior to the joining,either on the planar face 4 of the donor substrate 1, or on the planarface 6 of the carrier substrate 7 to be joined, or on both. Thisintermediate layer consists, for example, of silicon oxide, siliconnitride or polycrystalline silicon and has a thickness of between a fewnanometers and a few microns. The intermediate layer may be producedaccording to the various techniques known from the prior art, such asoxidation or nitridation heat treatments, chemical depositions (PECVD,LPCVD, etc.), etc. This intermediate layer, when it is polycrystalline,is configured, in terms of thickness, for example, to retain itspolycrystalline properties upon completion of the various heattreatments of the preparation process.

Upon completion of this joining step, the assembly shown in FIG. 1C isobtained, comprising the two associated substrates, the planar face 6 ofthe carrier substrate 7 adhering to the planar face 4 of the donorsubstrate 1.

The assembly is then treated to detach the thin layer 3 of ferroelectricmaterial from the donor substrate 1, for example, by cleavage at thelevel of the embrittlement plane 2.

This detachment step may thus comprise the application of a heattreatment in a temperature range of the order of 80° C. to about 300° C.to allow the transfer of the thin layer 3 to the carrier substrate 7.Instead of or in addition to the heat treatment, this step may comprisethe application of a blade, jet of gaseous or liquid fluid, or moregenerally any mechanical force to the embrittlement plane 2.

Following the detachment step, the structure 9 shown in FIG. 1D isobtained. This structure 9 comprises the thin layer 3 of ferroelectricmaterial comprising a first, free face 8 and a second planar face 4arranged on the carrier substrate 7. The thin layer 3 may equallyexhibit a single-domain or multi-domain character.

FIGS. 2A to 2D show a second embodiment of the structure 9, particularlysuitable for producing a heterogeneous structure 9, in which the thinlayer 3 exhibits a coefficient of thermal expansion that is quitedifferent from that of the carrier substrate 7, for example, exhibitinga difference of more than 10%.

With reference to FIG. 2A, the donor substrate 1 is, in this case,composed of a thick layer of ferroelectric material 1 a based on alkalimetal, possessing the same properties as those described for the bulkblock of ferroelectric material in relation to the first embodiment, andof a handling substrate 1 b.

The handling substrate 1 b advantageously consists of a material (or ofa plurality of materials) giving it a coefficient of thermal expansionclose to that of which the carrier substrate 7 is composed. The term“close” means that the difference in coefficient of thermal expansion ofthe handling substrate 1 b and of that of the carrier substrate 7 isless, as an absolute value, than the difference in thermal expansion ofthe bulk block of ferroelectric material and of that of the carriersubstrate 7.

Preferentially, the handling substrate 1 b and the carrier substrate 7have an identical coefficient of thermal expansion. When joining thedonor substrate and the carrier substrate, a structure capable ofwithstanding a heat treatment at a relatively high temperature isformed, which temperature may even exceed the Curie temperature of thethick layer of ferroelectric material 1 a based on alkali metal. For thesake of ease of implementation, this may be obtained by selecting thehandling substrate 1 b so that it consists of the same material as thatof the carrier substrate 7.

To form the donor substrate 1 of this embodiment, a bulk block offerroelectric material is joined to the handling substrate 1 bbeforehand, for example, by means of a molecular adhesive bondingtechnique such as has been described previously. Next, the layer offerroelectric material 1 a is formed by thinning, for example, bymilling and/or chemical-mechanical polishing and/or etching. Beforejoining, the formation of an adhesion layer (for example, by depositionof silicon oxide and/or silicon nitride) on one and/or another of thefaces placed in contact may have been envisaged. The joining maycomprise the application of a low-temperature heat treatment (forexample, between 50° C. and 300° C., typically 100° C.) making itpossible to strengthen the bonding energy sufficiently to allow thefollowing step of thinning.

The handling substrate 1 b is chosen to have a thickness substantiallyequivalent to that of the carrier substrate 7. The thinning step isperformed such that the thick layer 1 a has a thickness that is lowenough for the stresses generated during the heat treatments applied inthe rest of the process to be of reduced intensity. At the same time,this thickness is high enough to be able to take the thin layer 3 or aplurality of such layers therefrom. This thickness may be, for example,between 5 and 400 microns.

The following steps of the process of this second implementation areequivalent to those of the steps described in the first embodiment.Atomic species are implanted within the thick layer of ferroelectricmaterial 1 a in order to produce an embrittlement plane 2, whichdemarcates the separation of the thin layer 3 from the remaining portion5 of the donor substrate 1, as shown in FIG. 2B. This step is followedby a step of joining the donor substrate 1 to the carrier substrate 7,as shown in FIG. 2C. Next, the thin layer 3 is detached from the rest ofthe remaining portion 5 of the donor substrate 1 in order to obtain thestructure 9 shown in FIG. 2D.

This embodiment is advantageous in that the assembly formed of the donorsubstrate 1 and of the carrier substrate 7 may be exposed to atemperature substantially higher than that applied in the context of thefirst embodiment, without risking the uncontrolled splitting of one ofthe substrates or the delamination of the donor substrate 1 from thethin layer 3. The balanced structure, in terms of coefficient of thermalexpansion of this assembly, thus makes it possible to facilitate thestep of detaching the thin layer 3 by exposing the assembly to arelatively high temperature, for example, between 100° C. and 500° C. ormore. This temperature may be higher than the Curie temperature of thethin layer 3, since the single-domain character of this thin layer 3could be re-established in the preparation steps, the description ofwhich follows.

Whichever implementation is chosen, and as specified in the introductionof this patent application, steps of preparing the thin layer 3 aresubsequently necessary in order to improve the crystal, surface andadhesion quality of the thin layer 3.

The process for preparing the thin layer 3, illustrated by FIGS. 3A-3C,includes a first heat treatment (FIG. 3A) applied to the transferredthin layer 3. This heat treatment allows crystal defects that may havebeen generated in the preceding implantation and splitting steps to becorrected. In addition, it contributes towards consolidating the bondingbetween the thin layer 3 and the carrier substrate 7. According to thepresent disclosure, this first heat treatment brings the structure to atemperature higher than the Curie temperature of the thin layer 3, whileof course not exceeding the melting point of this layer, for a durationtypically of between 30 minutes and 10 hours. This heat treatment ispreferably carried out by exposing the free face of the thin layer 3 toan oxidizing or neutral gaseous atmosphere, i.e., without covering thisface of the thin layer with a protective layer.

This first heat treatment is carried out at a temperature higher thanthe Curie temperature of the thin layer 3, which then temporarily losesits ferroelectric properties. When the temperature of the thin layer 3falls back below its Curie temperature as it cools, new ferroelectricproperties are obtained. These new ferroelectric properties aregenerally different from the initial ones and may vary locally, thusgiving the thin layer a multi-domain character.

Upon completion of this first heat treatment, the thin layer 3 possessesa multi-domain character. However, the thin layer 3 possesses a bettercrystal quality and a better degree of adhesion with the carriersubstrate 7 than an identical thin layer that has been treated by afirst heat treatment carried out at a temperature lower than its Curietemperature.

The purpose of the next step is to re-establish the single-domaincharacter of the thin layer 3. This step includes introducing, after thefirst heat treatment, protons (FIG. 3B), for example, protons fromhydrogen, into the thin layer 3 followed by applying a second heattreatment (FIG. 3C) of the thin layer 3 at a temperature lower than itsCurie temperature.

The purpose of introducing protons is to generate a positive chargegradient in the thin layer 3 in order to induce an internal electricfield that exceeds the internal coercive field. In this way, thematerial is oriented in the direction of this internal field in order tomake it single domain on the scale of the thin layer 3.

Protons may be introduced in various ways. It may be a proton exchangecarried out, for example, by immersing at least the thin layer 3 in abath of benzoic acid at a temperature typically between 200° C. and 300°C. for 10 minutes to 30 hours.

Protons may also be introduced by means of ion implantation or by meansof plasma implantation.

The second heat treatment makes it possible to facilitate thereorientation of the internal electric field by promoting the diffusionof the protons, which results in the charge gradient being improved, andby decreasing the value of the coercive field.

Reference may be made to the documents “Discussion of domain inversionin LiNbO₃, L. Huang, N A F Jaeger, Applied physics letters, 1994,” and“Single-domain surface layers formed by heat treatment ofproton-exchanged multi-domain LiTaO₃ crystals, K. Nakamura and A.Tourlog, Applied physics letters, 1993,” for more detail on themechanisms involved in this heat treatment, and reasons why these stepsallow a single-domain layer to be provided.

The second heat treatment brings the structure to a temperature lower byless than 100° C., preferably less than 50° C., or less than 10° C. thanthe Curie temperature of the thin layer 3, preferably for a duration ofbetween 30 minutes and 10 hours. This second heat treatment, just likethe first heat treatment, is preferentially performed by exposing thefree face of the thin layer 3 to an oxidizing or neutral gaseousatmosphere.

Upon completion of this second heat treatment, the thin layer 3 thenexhibits a single-domain character.

The preparation process may also include, just after the first heattreatment or after the second heat treatment, thinning of the thin layer3. This thinning may correspond to polishing of the first, free face 8of the thin layer 3, for example, by means of mechanical,chemical-mechanical and/or chemical etching thinning techniques. Itmakes it possible to prepare the free face 8 so that it has littleroughness, for example, less than 0.5 nm RMS 5×5 μm by atomic forcemeasurement (AFM) and to remove a surface portion of the first, freeface 8 of the thin layer 3 that is liable to contain residual defects.

Exposing the thin layer to temperatures exceeding its Curie temperatureafter this second heat treatment will be avoided so as to maintain thesingle-domain property that has been re-established.

Of course, the present disclosure is not limited to the describedembodiments and variants thereof may fall within the scope of theinvention such as defined by the claims.

The invention claimed is:
 1. A process for preparing a thin layer offerroelectric material based on alkali metal, exhibiting a Curietemperature, comprising: transferring the thin layer of ferroelectricmaterial based on alkali metal from a donor substrate to a carriersubstrate using a transfer technique including implanting atomic speciesinto the donor substrate to produce an embrittlement plane delimitingthe thin layer, which has a first, free face and a second face that isdisposed on the carrier substrate after transferring the thin layer tothe carrier substrate, wherein the donor substrate has a coefficient ofthermal expansion close to that of the carrier substrate; applying afirst heat treatment to the transferred thin layer at a temperaturehigher than the Curie temperature, the thin layer of ferroelectricmaterial based on alkali metal exhibiting a multi-domain character uponcompletion of the first heat treatment; introducing protons into thethin layer after the first heat treatment; and after introducing theprotons into the thin layer, applying a second heat treatment to thethin layer at a temperature lower than the Curie temperature andgenerating an internal electric field resulting in the thin layer offerroelectric material based on alkali metal being made single domain.2. The process according to claim 1, wherein transferring the thin layerfrom a donor substrate to a carrier substrate includes joining the donorsubstrate to the carrier substrate and detaching the thin layer at thelevel of the embrittlement plane.
 3. The process according to claim 1,wherein the introducing of the protons into the thin layer is achievedby way of ion implantation or by way of plasma implantation.
 4. Theprocess according to claim 1, wherein each of the first heat treatmentand the second heat treatment are carried out under an oxidizing orneutral atmosphere.
 5. The process according to claim 1, wherein thefirst heat treatment is carried out for a duration of between 30 minutesand 10 hours.
 6. The process according to claim 1, wherein the thinlayer of ferroelectric material exhibits a 42° RY crystal orientation.7. The process according to claim 1, wherein the carrier substratecomprises silicon.
 8. The process according to claim 1, wherein thesecond heat treatment is carried out at a temperature less than 100° C.from the Curie temperature and for a duration of between 30 minutes and10 hours.
 9. The process according to claim 8, wherein the second heattreatment is carried out at a temperature less than 10° C. from theCurie temperature.
 10. The process according to claim 1, wherein theintroducing of the protons into the thin layer is achieved by way ofproton exchange.
 11. The process according to claim 10, wherein theproton exchange is carried out by immersing at least the thin layer in abath of benzoic acid at a temperature between 200° C. and 300° C. for 10minutes to 30 hours.
 12. The process according to claim 1, wherein thedonor substrate comprises a ferroelectric material based on lithium. 13.The process according to claim 12, wherein the donor substrate comprisesLiTaO₃ or of LiNbO₃.
 14. The process according to claim 1, furthercomprising polishing the first, free face of the thin layer.
 15. Theprocess according to claim 14, wherein the polishing is carried outafter the first heat treatment or after the second heat treatment. 16.The process according to claim 1, wherein the implanted atomic speciesare hydrogen and/or helium ions.
 17. The process according to claim 16,wherein transferring the thin layer from a donor substrate to a carriersubstrate includes joining the donor substrate to the carrier substrateand detaching the thin layer at the level of the embrittlement plane.18. The process according to claim 17, wherein the introducing of theprotons into the thin layer is achieved by way of ion implantation or byway of plasma implantation.
 19. The process according to claim 17,wherein the introducing of the protons into the thin layer is achievedby way of proton exchange.
 20. The process according to claim 19,wherein the proton exchange is carried out by immersing at least thethin layer in a bath of benzoic acid at a temperature between 200° C.and 300° C. for 10 minutes to 30 hours.